The present invention relates to an apparatus and method for producing nitrogen from a single column nitrogen generator. More particularly, the present invention relates to such an apparatus and method in which refrigeration is added by waste expansion. Evan more particularly, the present invention relates to such an apparatus and method in which a liquid product is produced by provision of additional air expansion.
There are many prior methods and apparatus by which nitrogen is produced in a single column known as a single column nitrogen generator. In such processes, air is compressed and then purified to remove carbon dioxide and moisture as well as potentially dangerous hydrocarbons. The compressed and purified air is then cooled in a main heat exchanger to a temperature suitable for its rectification which is normally at or near the dew point of air at the particular compression pressure. The air is then introduced into a distillation column to produce a nitrogen rich tower overhead and an oxygen enriched liquid column bottoms. The tower overhead is condensed and returned to the column for reflux purposes. A remaining part of the tower overhead may then be taken as a gaseous nitrogen product which fully warms in the main heat exchanger, thereby helping to cool the incoming air. Nitrogen of lesser purity can also be taken and passed through the main heat exchanger as a waste stream. Such waste stream can be used to regenerate the purifier.
In any air separation scheme, the power consumed is a very important consideration. In U.S. Pat. No. 4,966,002, a waste stream composed of the liquid column bottoms is valve expanded and then used as a coolant in the head condenser. The waste stream is then divided into two parts. One part of the waste stream is partially warmed and then expanded and the other part of the waste stream is recompressed and returned back to the column. The compression can either take place at the warm or cold end temperatures of the main heat exchanger. Increased efficiency has been realized by removing a liquid stream from the column having a higher nitrogen content than the column bottoms. Such liquid stream is then also valve expanded and introduced into the head condenser to act as a secondary coolant to help condense tower overhead for reflux purposes. The waste stream is partially warmed, expanded with the performance of work and then discharged from the main heat exchanger. The liquid stream that acts as a secondary coolant is recompressed after having served as a coolant, cooled back to its dew point temperature and reintroduced into the column.
In plants, such as those described above, it is difficult to supply sufficient refrigeration to generate liquid directly from the column. This is because the work of expansion, above that required to compress the recirculated stream must be discharged from the process as heat. Thus, in another patent, a nitrogen liquefier is integrated into the process in order to generate the liquid. The disadvantage of such integration is in the added expenses involved in supplying the equipment for the nitrogen liquefier.
As will be discussed, the present invention provides a method of producing a liquid nitrogen product from a single column nitrogen generator that is far simpler and more capital efficient than providing a separate nitrogen liquefier.